1. Field of the Invention
The present invention relates to an electric potential sensor, and more particularly, to a feedback type electric potential sensor which measures the electric potential of an object by causing the electric potential of the sensor to be equal to the electric potential of the object.
2. Description of the Related Art
Recently, many devices (for example a PPC copying machine or a laser printer, an electrostatic dust remover) utilize static electricity. In such devices, a sensor for measuring electric potential at a charged part is essential in order that the electric potential at the charged part can be maintained at a desired value.
FIG. 4 is a schematic diagram showing an example of such an electric potential sensor. This sensor detects an electric potential Vt of an object OBJ in non-contact manner. The sensor includes a pickup device Pu which calculates the difference between the electric potential Vt of the object OBJ and an electric potential Vdet at a detection node DET and controls the base current IB of transistor Tr so that the difference between the electric potential at the detection node DET and the electric potential Vt of the object OBJ becomes 0. The electric potential Vdet at the detection node DET is divided by resistances Ra and Rb (which are situated between the detection node DET and ground GND) and is output at output node OUT. This output is indicative of the electric potential of the measuring object OBJ is to be measured.
There are several drawbacks to the prior art system. Because one side of the power source HV is grounded, only either a positive electric potential or, alternatively, a negative potential can be measured. Further, due to the influence of the leakage current of the transistor Tr, and the like, it is difficult to accurately measure a 0 V electric potential which may be a threshold value in a measuring range of the sensor. For example, in a sensor used to measure a positive electric potential, a 0 V electric potential and negative electric potentials cannot be distinguished, etc.
To overcome these problems, there has been proposed an electric potential sensor shown in FIG. 5 which can measure both positive and negative electric potentials. In this sensor, two power sources HV1 and HV2, and two transistors Tr1 and Tr2, are provided. The mutual connection node of the power sources HV1 and HV2 is grounded, and resistances Rc and Rd are provided between grounded GRD and the detection node DET. As a result, both positive and negative electric potentials can be measured, and 0 electric potential can be accurately measured. However, the structure of this sensor is complex and its cost is high.